Printing plates may be imaged on a plate-making machine and then transferred to a printing press. Once on the printing press, the images from the printing plates are transferred to paper or other suitable substrates. It is important that images printed using a printing press be properly aligned with the substrate on which they are printed. Obtaining such alignment typically involves:                carefully aligning a reference edge of a printing plate with pins or other features on the plate making machine;        detecting one reference point on an orthogonal edge of the printing plate (i.e. orthogonal to the reference edge) at a known distance from the reference pins;        imaging the printing plate; and        using the reference edge and the orthogonal edge reference point to align the printing plate on a drum of the printing press.        
One common technique of aligning the printing plate on the drum of a printing press involves using the reference edge and the orthogonal edge reference point to align the printing plate on a punching machine and punching registration holes in the printing plate. The printing plate may then be aligned on the drum of the printing press with registration pins that project through the registration holes.
Traditionally mechanical alignment pins have been used to align the plate to be imaged to the drum of a platesetter. This is not a flexible arrangement. The pins have to be mounted in predetermined positions. There are also reliability challenges in consistently and accurately loading the plate into contact with the pins. It is also difficult to define sets of pins that allow a wide range of plate formats to be imaged whilst not interfering with one another.
There is therefore a need for a alignment mechanism not based on mechanical locating pins. A number of these have been proposed.
Techniques for detecting an edge of a printing plate, and any associated skew in the plate are disclosed in U.S. Pat. No. 6,321,651 (Tice et al.) and in U.S. Pat. No. 6,318,262 (Wolber et al.). In these patents edge detection sensors are employed in the load path to an imaging drum upon which a printing plate is imaged. U.S. Pat. No. 4,881,086 (Misawa) describes a laser recorder with sheet edge detection. The edge detection works on the principle of the difference in reflectivity between that of the sheet and that of the drum on which it is carried.
European Patent Application EP 1 081 458 A2 (Elior et al.) describes an apparatus for detecting a plate edge including an optical printhead for illuminating a plate system with a light beam having a focal point generally at the plate, a detector for measuring the intensity of the light reflected from the plate system, and means for altering the relative position of the edge of the plate and the light beam. EP 1 081 458 A2 also teach the use of this apparatus to determine a skew angle of the plate mounted on a plate support surface.
U.S. Pat. No. 6,815,702 (Kiermeier et al.) describes detecting the edge of a printing plate mounted on an external drum by using a light source and light sensor to measure the difference in reflectivity between the plate and drum surface. Two grooves formed in the drum surface are used to increase the difference in reflectivity between the plate and the drum. A difference in the positions where the plate covers each of the grooves is determined and compared against predetermined value to determine if a skewed plate is mounted on the drum.
U.S. Pat. No. 4,876,456 (Isono et al.) describe using photosensors having light emitting elements and light receiving elements disposed in a path for carrying a photosensitive film. The light emitting elements emit periodic pulsed light. When the film is present at the locations of the photosensors, the periodic pulsed light is reflected by the film to enter the light receiving elements thereby detecting the presence of the film. The activation time and activation interval of the light emitting elements are determined so that accumulated exposure value in each portion on the film is less than an upper limit value of accumulated exposure value of the photosensitive film.
In commonly-assigned U.S. patent application Ser. No. 11/204,223 an edge detection system is described, based on using a CCD camera to image the edges of a printing plate perpendicular to the sub-scan direction. Based on the information so obtained, the image data is then adjusted to compensate for any misalignment between the plate and the drum on which it is loaded.
The problem with front side illumination of printing plates on a platesetter is that the bevelled nature of the aluminium plate edge makes the determination of the outer mechanical limit of that edge rather difficult. This happens because both the flat top surface of the plate and the bevelled edge are likely to reflect the illuminating light when a CCD camera is used to image the edge.
A further limitation as regards front side illumination arises in the use of light sensitive plates, in which case light that could expose the emulsion has to be avoided.
There is therefore a need in the printing industry for a method and apparatus capable of consistently and automatically determining the outer mechanical edge of the plate that is to be imaged.